Flaw detection system using microwaves

ABSTRACT

A method and apparatus for the detection of flaws in pieces of lumber or like material in which there may be wide variations in density within each piece, or among the pieces being examined. Lumber being processed is moved past two adjacent microwave detectors positioned to receive microwave radiation transmitted through the moving lumber from a conventional microwave source, and the outputs from the detectors are amplified and compared in a differential amplifier, the presence of flaws resulting in difference signals which are used to generate pulses for controlling a lumber processing device or for storage in a memory device and use in subsequent processing steps.

United States Patent [191 Bennion etal.

[ 1 FLAW DETECTION SYSTEM USING MICROWAVES [75] Inventors: Don U. Bennion; Leonard R. West,

both of Salt Lake City, Utah [73] Assignee: Industrial Development Design, Salt Lake City, Utah [22] Filed: Mar. 26, 1973 [21] Appl. No.: 344,932

[52] U.S. Cl 324/585 A, 73/104, 73/159, 144/312, 340/248 [51] Int. Cl G0lr 27/04 [58] Field of Search. 324/58.5 A, 58A, 95, 58.5 B; 246/169 D; 340/248; 73/104, 159; 144/312 [11] 3,810,005 [4 May 7,1974

FOREIGN PATENTS OR APPLICATIONS 1,234,764 5/1960 France .Q 246/169 D Primary Examiner-Stanley T. Krawczewicz Attorney, Agent, or Firm-Fulwider, Patton, Rieber, Lee & Utecht 57 ABSTRACT A method and apparatus for the detection of flaws in pieces of lumber or like material in which there may be wide variations in density within each piece, or

among the pieces being examined. Lumber being processed is moved past two adjacent microwave detectors positioned to receive microwave radiation transmitted through the moving lumber from a conventional microwave source, and the outputs from the detectors are amplified and compared in a differential amplifier, the presence of flaws resulting in difference signals which are used to generate pulses for controlling a lumber processing device or for storage in a memory device and use in subsequent processing steps.

28 Claims, 2 Drawing Figures 1411/15 C/ECW/T (22 COMM/7E? I Min/DIV a 4105/:

DEV/CE MTENTEBIAY 7 m4 SHEU 2 UF 2 mN N W wmwwwwmg FLAW DETECTION SYSTEM USING MICROWAVES BACKGROUND OF THE INVENTION This invention relates generally to systems for the detection of flaws in lumber and, more particularly, to lumber flaw detection systems using microwave radiation. Such systems are used to supply information for automatically marking or cutting the lumber into desired lengthsfree of flaws.

In the prior art, short'wave electromagnetic radiation, widely known as microwave radiation, has been used to detect flaws or non-homogeneous zones in electrically non-conducting material, particularly glass. The microwave radiation is either transmitted through or reflected from the material being examined, and a microwave receiver is used to detect variations in the transmitted or reflected-radiation caused by flaws in the material as it is moved past the receiver.

Common defects in lumber, such as knots, pitch, and wind shake, are non-homogeneous zones which can be readily detected using a microwave technique. However, because lumber is not, in general, as homogeneous as many synthetic materials, such as glass, prior art microwave techniques are not directly transferable to lumber flaw detection systems. Because of the widely varying moisture content of lumber, the density of the lumber may vary widely within a single piece as well as from piece to piece. These density variations show up as variations in the received microwave radiation,

making the detection of flaws in the lumber very difficult, if. not impossible, using prior art methods. One possible solution to this problem is to filter out the density variations, which will generally be more gradual than the variations due to flaws, by employing, for example, some form of capacitance coupling in circuitry associated with the receiver.-This is not a completely satisfactory solution, however, since the speed of the lumber with respect to the receiver then becomes a critical factor in the use of the technique.

Ideally, a flaw detection system for lumber should be insensitive to normal variations in density of the lumber from piece to piece and within single pieces, while still remaining sufficiently sensitive to changes in density due to flaws in the lumber. Furthermore, this insensitivity to normal density variations should not be achieved at the expense of a lower processing speed. The present invention meets all of these needs.

SUMMARY OF THE INVENTION The present invention resides in an improved method and apparatus for the detection and location of flaws in lumber and like material, using microwave radiation. Basically, the apparatus of the invention includes a source of microwave radiation, at least two microwave detectors, a differential amplifier for detecting a difference in output between the detectors, and a means for coupling the differential amplifier to a control or storage device used for subsequent processing of the lumber. i

Normal variations in the density of the lumber, because of their gradual nature, will generally have equal effects on adjacent detectors and will produce very little difference signaLSmall flaws such as knots, on the other, hand, will affect the output of only one detector at a time, and will result in a momentary difference signal which can be used to operate a lumber marking or cutting device, or stored for subsequent use in processing the lumber.

The method of the present invention basically includes the steps of transmitting microwave radiation through the lumber, measuring received radiation at at least two adjacent positions, sensing differences in the measured radiation at the adjacent positions, and generating therefrom control signals for use in processing the lumber.

More specifically, in a presently preferred embodiment of the invention, two microwave detectors are employed, the output of each being amplified and fed to a variable gain differential amplifier, the output of which is proportional to the difference between the outputs of the two detectors. Since the detectors are positioned close together, gradual variations in lumber density produce no difference signal, or a negligibly small difference signal, but small flaws in the lumber produce substantial difference signals at the output of the differential amplifier. An absolute value circuit is included, following the differential amplifier, in order that both positive and negative difference signals will bedetected. Then, a level sensing circuit serves a filtering function by rejecting difference signals below a certain level, and passing others on to a trigger circuit which, in cooperation with the level sensing circuit, produces pulse signalsto be stored or used to operate lumber processing equipment.

It can be readily seen that the invention provides a new and useful tool for the detection and location of flaws in lumber. The invention overcomes the'disadvantages of prior art microwave flaw detection systems as applied to lumber and like materials, in that small flaws can be conveniently distinguished from normal fluctuations in density without imposing any limitation on lumber processing speed. Other aspectsand advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.

embodying the invention; and

FIG. 2 is a more detailed electricalschematic diagram of the system shown in FIG. 1.

As shown in the drawings for purposes of illustration, the invention is embodied in an improved system using microwave radiation for the detection of flaws, such as knots, pitch, and wind shake, in lumber. It will be understood that, while the invention is particularly well suited for the detection of flaws in lumber, it could also be used for the detection of flaws in other like materials, lacking perfect'homogeneity.

In accordance with the present invention, as illustrated in FIG. 1, a piece of lumber 10 being examined for flaws is passed between a microwave oscillator source 11 and two microwave detectors l2 and 13, the outputs of which are amplified by two corresponding amplifiers l4 and 15, and connected to a differential amplifier 16, which produces an output proportional to the difference between the outputs of the two microwave detectors. Since the detectors 12 and 13 are disposed close together in the line of motion of the lumber l0, gradual density variations along the length of the lumber will generally affect both detectors equally, and

. will result in a negligibly small difference signal, if any,

at the output of the differential amplifier 16. A small flaw, on the other hand, such as the one shown at 17, will affect the output of only one of the detectors 12 and 13 at a time and will produce a relatively large difference signal at the output of the differential amplifier 16. Thus, the system is capable of detecting flaws in material having normal inherent density fluctuations.

More specifically, the microwave oscillator 11 is a conventional microwave source, of either the klystron or the solid state diode variety, generating microwave radiation having a wavelength typically in excess of one centimeter. Flaws detected by the system are typically larger than one-eighth inch (approximately 3 mm) in diameter. The microwave detectors 12 and 13 are positioned adjacent to each other in the path of the radiation transmitted through the lumber 10, and include microwave horns and diode detectors of conventional design. 1

The amplifiers 14 and 15, to which the outputs of the detectors 12 and 13 are connected, are single-ended operational amplifiers. Since these devices have a characteristically high input impedance and low output impedance, they serve the dual function of amplification and impedance transformation. The differential amplifier 16 is also an operational amplifier, connected to have a variable gain so that the sensitivity of the detection system may be varied.

The output from the differential amplifier 16 is connected to an absolute value circuit 18 providing a positive output signal equal in magnitude to that of its input signal but without regard to the sign of the input signal. The output from the absolute value circuit 18 is, in turn, connected to a level sensing circuit 19, which serves a filtering function in that it rejects difference signals below a certain selectable threshold value. Difference signals of sufficient magnitude to activate the level sensing circuit 19 are conveyed to a trigger circuit 20, which, in cooperation with the level sensing circuit, produces signals inpulse form to operate a lumber markingdevice 21, or other'lumber processing device, or to be stored in computer memory and logic elements 22 for subsequent use in processing the lumber 10.

Referring now to FIG. 2, there is shown by way of example and not by way of limitation, a detailed schematic diagram of a presently preferred embodiment of the invention. The amplifiers 14 and and the differential amplifier 16 each include an operational, or d.c. amplifier having characteristically high gain, high input impedance, and low output impedance. Such operational amplifiers are typically available in integrated circuit form. In a presentlypreferred embodiment of the invention, each of the three amplifiers 14, 15 and 16 utilizes one-half of a dual operational amplifier integrated circuit chip, device type MC1437L, manufactured by Motorola Semiconductor Products, Inc., Phoenix, Ariz.

The first operational amplifier 14 has an inverting input terminal 25, a non-inverting input terminal 26, an output terminal 27, a 8+ power supply terminal 28, a 8- power supply terminal 29, two input lag terminals 30 and 31, and an output lag terminal 32. The second operational amplifier 15 has an inverting input terminal 34, an non-inverting input terminal 35, an output terminal 36, a 8+ power supply terminal 37, a B power supply terminal 38, two input lag terminals 39 and 4t), and an output lag terminal 41. Similarly, the differential amplifier 16 has an inverting input terminal 43, a non-inverting input terminal 44, an output terminal 45, a 8+ power supply terminal 46, a B- power supply terminal 47, two input lag terminals 48 and 49, and an output lag terminal 50.

Both the amplifiers l4 and 15 are connected in a non-inverting configuration, i.e., the output has the same sign as the input. The output of the first microwave detector 12 is applied between the non-inverting input terminal 26 of amplifier 14 and ground, a variable feedback resistor R51 connects the output terminal 27 to the inverting input terminal 25, and another resistor R52 is connected between the inverting input terminal 25 and ground. In this configuration, the closed loop gain, i.e., the ratio of the voltage at the output terminal 27 to the voltage at the non-inverting input terminal 26 is positive and is determined primarily by the values of the resistors R51 and R52. Power is'supplied to the amplifier 14 through the power supply terminals 28 and 29, and frequency compensation of the amplifier is provided by a resistor R53 and a capacitor C54 connected in series between the input lag terminals 30 and 31 and a further capacitor C55 connected between the output terminal 27 and the output lag terminal 32.

The second amplifier 15 is connected in a similar fashion, i.e., the output from the second microwave detector 13 is applied between the non-inverting input terminal 35 and ground, a variable feedback resistor R56 is connected between the output terminal 36 and the inverting input terminal 34, and another resistor R57, is connected between the inverting input terminal 34 and ground. Again, power supplied to the amplifier 15 through the power supply terminals 37 and 38, and frequency compensation is provided by a resistor R58 and a capacitor C59 connected in series between the input lag terminals 39 and 40, and a further capacitor C61 connected between the output terminal 36 and the output lag terminal 41.

Output from the amplifier 14 is connected from the output terminal 27 through a resistor R62 to the inverting input terminal 43 of the differential amplifier 16, and output from the amplifier 15 is connected from the output terminal 36 through a resistor R63 to the noninverting input'terminal 44 of the differential amplifier.

A feedback resistance comprising two resistors R64 and R65 in series is connected between the output terminal 45 and the inverting input terminal 43. Another resistance consisting also of two resistors R66 and R67 in series is connected between the non-inverting input terminal 44 and ground, and a variable resistor R68 is connected between the junction point of resistors R64 and R65, and the junction point of resistors R66 and R67. In this configuration, the differential amplifier 16 produces an output signal between its output terminal 45 and ground which is proportional to the difference between the output signals of amplifiers 14 and 15, and the variable resistor R68 operates to vary the gain of the differential amplifier. As in the other amplifiers, power is supplied to the differential amplifier 16 through the power supply terminals 46 and 47, and frequency compensation is effected by means of a resistor R69 and a capacitor C71 connected in series between the input lag terminals 48 and 49, and a further capacitor C72 connected between the output terminal 45 and the output lag terminal 50.

operational amplifier 74, which, like the others, has an inverting input terminal 75, a non-inverting input terminal 76, an output terminal 77, power supply terminals 78 and 79, input lag terminals 81 and 82, and an output lag terminal 83. A feedback resistor R84 is connected between the output terminal 77 and the inverting input terminal 75, and frequency compensation is provided by a resistor R85 and a capacitor C86 connected in series between the input lag terminals 81 and 82, and a further capacitor C90 connected between the output terminal 77 and the output lag terminal 83.

The output of the differential amplifier 16 is connected to the absolute value circuit through a network including three diodes 87, 88 and 89, two resistors R91 and R92, and a voltage divider consisting of an additional resistor R93 connected to the B+ voltage, and a low-resistance potentiometer R94 connecting the resistor R93 to ground.

The output terminal 45 of the differential amplifier 16 is connected through the resistor R91 to the inverting terminal 75, and is also connected to the anode of the first diode 87, the cathode of which is connected to the anode of the second diode 88. The cathode of the second diode 88 is, in turn, connected through the resistor R92 to the inverting terminal 75. Finally, the sliding contact of the potentiometer R94 is connected to the anode of the third diode 89, the cathode of which is connected both to the non-inverting input terminal 76 and to the junction point between the first and second diodes 87 and 88.

In a presently preferred embodiment of the invention, the feedback resistor R84 is IOkO, and the resistors R9 l and R92 are 10k!) and SkfLrespectively. With these values of resistance, the absolute value circuit 18 acts as an inverter with a gain of unity when negative signals are presented to it, and as a unity-gain amplifier for positive signals. For negative input signals, the only effective path is through the resistor R91 to the inverting input terminal 75, since other possible paths are blocked by the first diode 87. The non-inverting, or positive terminal 76 is held as close to ground potential as possible by adjustment of the potentiometer 94, and, for negative input signals, the circuit is effectively connected in a conventional inverting configuration for operational amplifier, the closed loop gain being primarily determined by the ratio of the resistances R84 and R91, i.e., -l.

For positive inputs, there are three possible paths: through the resistor R91 to the inverting input terminal 75, through the first diode 87 to the non-inverting input terminal 76, and through the first and second diodes 87 and 88 and the resistor R92 to the inverting input terminal 75. Using familiar principles of superposition, the closed loop gain can be computed for each path separately, and the values added to obtain the effective gain for positive input signals. For the first path, through the resistor R91 to the inverting terminal 75, the gain is -1, as was shown for negative input signals. The second path, through the first diode 87-to the non-inverting input terminal 76, is effectively a conventional noninverting configuration for operational amplifiers, with the input connected to the non-inverting input terminal, and the inverting input terminal 75 having a resistance to ground comprising the resistors R91 and R92 in parallel, i.e., a value of l0/3k1'l.

If the input voltage is designated e, and the voltage drop across the first and second diodes 87 and 88 are designated v, and v respectively, then the voltage applied to the non-inverting input terminal 76 through the second path is (e v and the closed loop gain for this voltage is: l (R84 10/3) +4.

For the third path, through the first and second diodes 87 and 88 and the resistor R92, the effective input voltage is (e v v and the closed loop gain is -R84/R92, or 2. Thus, the overall closed loop gain sums to +l if v, v,; and the absolute value circuit 18 outputs to the level sensing circuit 19 a positive signal equal in magnitude to that of the input signal.

The level sensing circuit 19 (FIG. 1) and the trigger circuit 20 (FIG. 1) include, as shown in FIG. 2, two unijunction transistors 96 and 97, two silicon controlled rectifiers (SCRs) 98 and 99, a NAND gate 101, and an npn transistor 102. The output signal from the absolute value circuit 18 is applied from the output terminal 77 through diode 103 and a resistor R104 to the emitter terminal of the first unijunction transistor 96, the baseone terminal of the unijunction transistor 96 is connected to ground through a resistor R105, and power is supplied to the base-two terminal of the unijunction transistor 96 through a resistor R106. In a presently preferred embodiment of the invention, the first uni junction transistor 96 will be turned on when theoutput from the absolute value circuit 18 is in the region of 50 to per cent of the power supply (B+) voltage.

Whenever the first unijunction transistor 96 is turned on by a sufficiently high output voltage from the differential amplifier 16, the first SCR 98 is also turned on, since the base-one terminal of the first unijunction transistor 96 is connected to the gate terminal of the first SCR 98, the anode terminal of which is supplied with power (B+) through a resistor R107, and the cathode terminal of which is connected to ground through a resistor R108. A capacitor C109 is connected between the anode terminal of the SCR 98 and ground, so that, when the SCR 98v is non-conducting, the capacitor Cl09 is charged from the power supply' When the SCR 98 is triggered, the capacitor C109 is discharged through the SCR 98 and through the resistor R108 to ground.

The pulse generated by the discharge of the capacitor C109 is fed through a coupling capacitor C111 to the emitter terminal of the second unijunction transistor 97, which has its base-one terminal connected to ground through a resistor R112 and its base-two terminal connected to the power supply (B+) through a resistor R113. In a voltage divider circuit consisting of a resistor R114 and a variable resistor R115 connected in series between the power supply (B+) and ground, the junction between the resistors R114 and R115 is connected to the emitter terminal of the second unijunction transistor 97, and the variable resistor R115 is adjusted so that a pulse from the capacitor C109 in the circuit of thefirst SCR 98 will, together with the voltage from the voltage divider circuit, apply a high enough voltage to the emitter of the second unijunction transistor 97 to momentarily turn that device on.

The second SCR 99 has its anode terminal connected to the power supply (B+) through two resistors R116 and R117 in series, the resistor R117 being variable and connected to the anode. A capacitor C118 is connected between the junction of the two resistors R116 and R117 and ground, the cathode of the SCR 99 is connected to ground, and the base-one terminal of the second unijunction transistor 97 is connected to the gate terminal of the second SCR 99. Thus, when the second unijunction transistor 97 is turned on, a pulse is applied to the gate terminal of the second SCR 99,-

which is turned on by the pulse. The capacitor C118, which is charged when the SCR 99 is non-conducting, is discharged through the resistor R117 and the SCR 99 when the latter is triggered, thus momentarily lowering the potential at the anode of the SCR 99.

The combination of the two unijunction transistors 96 and 97 and the two SCR 98 and 99 is designed to provide one short pulse to the NAND gate 101, which has one input connected to the anode terminal of the second SCR 99 and the other input terminal grounded.

'The logic of the NAND gate is such that ground voltage is a logical 1 and a positive voltage is a logical 0. Thus,.the inputs change momentarily from O and l to l and l respectively, when the anode potential of the SCR 99 falls, and the output of the NAND gate 101 momentarily changes from 1 to 0," i.e., a positive pulse is output. The output terminal of the NAND gate 101 is connected to the base terminal of the npn transistor 102, the emitter terminal of which is grounded and the collector terminal of which is connected to the power supply (8+) through a relay 119. When the NAND gate 101 is pulsed, the transistor 102 is momentarily forward biased, and the resulting collector current operates the relay 119. As shown by way of. example in FIG. 2,the relay 119 may operate a switch 121 in an a.c.'circuit 122 connected to supply power to the lumber marking device 21, or the operation of'the unijunction transistors 96 and 97 can be used to enter information into computer memory and logic circuits 22 (FIG. 1).

Normally, the RC time constant of the trigger circuit is adjusted, using the variable resistor R117, to keep the transistor 102 switched on for long enough for a flaw to pass the second of the two detectors l2 and 13. Thus, for a given maximum flaw size and lumber speed, only one pulse'would be generated during the transition of one flaw'past the detector system. For very large flaws, a second pulse will be generated as the last portion of the flaw passes the second detector. This will usually be a desirable result, since a small flaw can be removed with a single cut of a double-blade saw, while a large flaw will require two cuts.

In a presently preferred embodiment of the invention, the following values of resistance and capacitance may be used. It will be understood by those of ordinary skill in the electronics art that the invention is not limited to circuitry using the specific values set forth In the presently preferred embodiment, conventional components are employed throughout. As mentioned above, the operational amplifiers used in the embodiment are half-sections of dual operational amplifier modules designated MC1437L. The SCRs employed are designated 2N5060, the unijunction transistors 2N4891, the diodes 1N34A, the NAND gate MC673P, and the npn transistor 2N4921. Again it will be understood that the invention is not limited to the use of these specific components.

From the foregoing, it can be seen that the invention as described in detail for purposes of illustration, provides a new and useful tool for the detection and location of flaws in lumber and like material. Small flaws in moving pieces of lumber can be reliably detected and located, and can be conveniently distinguished from normal fluctuations in lumber density-without imposing any limitation on processing speed. It will also be seen that, while a particular form of the invention has been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited, except as by the appended claims.

' We claim:

1. For use with a system for detecting flaws in nonconducting material having at least one widely varying physical characteristic affecting the attenuation of microwave radiation transmitted through the material, the combination comprising: 7

at least two microwave detectors positioned to receive microwave radiation transmitted through and consequently attenuated by the material;

signal difference sensing means responsive to differences in output signals from said microwave detectors and non-responsive to variations in attenuation characteristics of the material affecting said microwave detectors substantially equally; and

electrical means for coupling said signal difference sensing means to means for controlling processing of the material, whereby'a flaw in the material produces a momentary difference in output signals from said microwave detectors as the flaw passes said detectors, and the difference signal is used to control processing of the material in such a fashion as to avoid the flaw.

2. A combination as set forth in claim 1, wherein there are two microwave detectors.

3. A combination as set forth in claim 1, wherein said electrical means include level sensing means for rejecting signal differences below a selectable threshold level.

4. A combination as set forth in claim 3, wherein said electrical means include trigger circuit means for generating control pulse signals in response to signal differ ences above said threshold levelf 5. A combination as set forth in claim 4, wherein said electrical means include absolute value generation means equally responsive to positive and negative signal differences.

6. A combination as set forth in claim 4, wherein there are two microwave detectors and the material is lumber.

7. Apparatus for detection of flaws in moving lumber of widely varying density, said apparatus comprising:

a source of microwave radiation positioned to irradiate the moving lumber;

at least two microwave detectors adjacently positioned to receive the radiation after transmission through the lumber and consequent attenuation and distortion by the presence of flaws; differential amplifier means for detecting and amplifying differences in output signals from said microwave detectors, said differences being generated as a result of flaws passing across one of said detectors and said differential amplifier means being nonresponsive to variations in the output signals due to variations in lumber density affecting said microwave detectors substantially equally; and electrical means for coupling said differential amplifier means to means for controlling processing of the lumber in such a fashion as to avoid'the flaws.

8. Apparatus as set forth in claim 7, wherein there are two microwave detectors.

11. Apparatus as set forthin claim 10, wherein said electrical 'meansinclude trigger circuit means for generating control pulse'signals in response to signal differences' above said threshold level.

12. Apparatus as set forth in claim 11, wherein there are two microwave detectors.

13. Apparatus as set forth in claim 12; wherein said electrical means include absolute value generation means equally responsive to positive and negative signal differences. I t

14. Apparatus as set forth in claim 13, wherein:

said source of microwave radiation generates electromagnetic radiation having a wavelength greater than 1 centimeter; and

the flaws to be detected exceed 3 millimeters in diameter. t i

15. Apparatus for detection offlaws in moving lumber of widely varying density, said apparatus comprism 5 source of microwave radiation positioned to irradiate the moving lumber;

first and second microwave detectors positioned'adjacently in the line of motion of the lumber to receive the radiation after transmission through the lumber and consequent attenuationand distortion by the presence of flaws;

first andsecond single-ended operational amplifiers for receiving and amplifying corresponding output signals from said first and second microwave detectors;

a differential operational amplifier for detecting and amplifying signal differences between the output signals from said first and second microwave detectors, said signal differences resulting from flaws passing across one of said detectors and said differential operational amplifier being non-responsive to variations in the output signals from said first and second single-ended operational amplifiers due to variations in lumber density affecting said first and second microwave detectors equally;

absolute value generation means for inverting negative signal differences but not positive signal differences;

signal level sensing means for rejecting those of said signal differences below a selectable threshold level and thereby rejecting variations in the output signals from said first and second single-ended operational amplifiers due to variations in lumber density affecting said first and second microwave detectors substantially equally; and

trigger circuit means for generating control pulse signals in response to those of said signal differences above said threshold level. i

16. Apparatus as set forth in claim 15, wherein:

said source of microwave radiation generates electromagnetic radiation having a wavelength greater than 1 centimeter; and

the flaws to be detected exceed 3 ameter. a

17. Apparatus as set forthin claim 15, wherein:

said level sensing means includes at least one unijunction transistor; and j said trigger circuit means includes at least one silicon controlled rectifier. j 18. A method for detecting flaws in moving lumber of widely varying density. comprising the steps of:

transmitting microwave radiation through the lumber; I e

measuring received radiation at at least two adjacent positions substantially equally affected by attenuamillimeters in dition variations caused by the widely varying lumber density;

sensing significantdifferences in the radiation measured at said adjacent positions, said differences resulting only from the presence of flaws in the lumber moving through said, adjacent positions; and a I generating in response to said significant differences control signalsfor use in processing the lumber.

19. A method as set forth in claim 18, wherein said step of sensing significant differences includes amplifying said differences. t

.20. A method asset forth in claim 19,wherein said step of sensing significant differences includes sensing the absolute magnitude of said differences and rejecting those of said differences below a selectable threshold level.

21. A method as-set forth in claim 20, wherein said step of generating control signals includes triggering generation of control pulse signals in response to those of said differences above the threshold level.

22. A method for detecting flaws in lumber of widely varying density, comprising the steps of:

transmitting radiation from a microwave oscillator source; moving the lumber through the microwave radiation;

radiation due to the widely varying lumber density;

detecting significant difference signals between outputs from the two detectors, said significant differ- 11 ence signals resulting only from the presence of flaws in the lumber moving across one of the detectors; and

generating in response to said significant difference signals control signals for use in processing the lumber.

23. A method as set forth in claim 22, wherein:

said step of transmitting utilizes a wavelength greater than 1 centimeter; and

the flaws to be detected exceed 3 millimeters in diameter.

24. A method as set forth in claim 22, wherein said step of detecting significant difference signals includes inverting all negative ones of the signal differences, thereby generating absolute values of the signal differences.

25. A method as set forthin claim 24, wherein said step of detecting significant difference signals includes sensing the absolute values of the signal differences and rejecting those below a selectable threshold level.

26. A method as set forth in claim 25, wherein said step of generating control signals includes triggering generation of control pulse signals in response to those of said significant difference signals above the threshold level.

27. A method for detecting flaws in lumber with widely varying density, comprising the steps of:

transmitting microwave radiation from a microwave oscillator source;

receiving the microwave radiation at two adjacent microwave detectors after transmission through the lumber;

moving the lumber between the microwave oscillator source and the two microwave detectors in a direction substantially at right angles to the direction of radiation;

amplifying output signals from the two microwave detectors in two corresponding single-ended operational amplifiers;

detecting and amplifying in an operational amplifier difference signals representative of the difference between the two output signals, said difference signals resulting from the momentary presence of flaws in the lumber moving across one of the detectors;

disregarding gradual fluctuations in lumber density,

said fluctuations resulting in practically equal effects on the microwave radiation received at the microwave detectors;

inverting all negative ones of said difference signals,

thereby generating absolute magnitudes of said difference signals;

sensing the absolute magnitudes of said difference signals and rejecting those of said difference signals below a selectable threshold level; and

generating control pulse signals for use in processing the lumber, in response to those of said difference signals above the threshold level.

28. A method as set forth in claim 25, wherein:

said step of transmitting utilizes a wavelength greater than 1 centimeter; and

the flaws to be detected exceed 3 millimeters in diameter. 

1. For use with a system for detecting flaws in non-conducting material having at least one widely varying physical characteristic affecting the attenuatiOn of microwave radiation transmitted through the material, the combination comprising: at least two microwave detectors positioned to receive microwave radiation transmitted through and consequently attenuated by the material; signal difference sensing means responsive to differences in output signals from said microwave detectors and non-responsive to variations in attenuation characteristics of the material affecting said microwave detectors substantially equally; and electrical means for coupling said signal difference sensing means to means for controlling processing of the material, whereby a flaw in the material produces a momentary difference in output signals from said microwave detectors as the flaw passes said detectors, and the difference signal is used to control processing of the material in such a fashion as to avoid the flaw.
 2. A combination as set forth in claim 1, wherein there are two microwave detectors.
 3. A combination as set forth in claim 1, wherein said electrical means include level sensing means for rejecting signal differences below a selectable threshold level.
 4. A combination as set forth in claim 3, wherein said electrical means include trigger circuit means for generating control pulse signals in response to signal differences above said threshold level.
 5. A combination as set forth in claim 4, wherein said electrical means include absolute value generation means equally responsive to positive and negative signal differences.
 6. A combination as set forth in claim 4, wherein there are two microwave detectors and the material is lumber.
 7. Apparatus for detection of flaws in moving lumber of widely varying density, said apparatus comprising: a source of microwave radiation positioned to irradiate the moving lumber; at least two microwave detectors adjacently positioned to receive the radiation after transmission through the lumber and consequent attenuation and distortion by the presence of flaws; differential amplifier means for detecting and amplifying differences in output signals from said microwave detectors, said differences being generated as a result of flaws passing across one of said detectors and said differential amplifier means being non-responsive to variations in the output signals due to variations in lumber density affecting said microwave detectors substantially equally; and electrical means for coupling said differential amplifier means to means for controlling processing of the lumber in such a fashion as to avoid the flaws.
 8. Apparatus as set forth in claim 7, wherein there are two microwave detectors.
 9. Apparatus as set forth in claim 8, wherein: said source of microwave radiation generates electromagnetic radiation having a wavelength greater than 1 centimeter; and the flaws to be detected exceed 3 millimeters in diameter.
 10. Apparatus as set forth in claim 7, wherein said differential amplifier means include level sensing means for rejecting signal differences below a selectable threshold level.
 11. Apparatus as set forth in claim 10, wherein said electrical means include trigger circuit means for generating control pulse signals in response to signal differences above said threshold level.
 12. Apparatus as set forth in claim 11, wherein there are two microwave detectors.
 13. Apparatus as set forth in claim 12, wherein said electrical means include absolute value generation means equally responsive to positive and negative signal differences.
 14. Apparatus as set forth in claim 13, wherein: said source of microwave radiation generates electromagnetic radiation having a wavelength greater than 1 centimeter; and the flaws to be detected exceed 3 millimeters in diameter.
 15. Apparatus for detection of flaws in moving lumber of widely varying density, said apparatus comprising: a source of microwave radiation positioned to irradiate the moving lumber; first and second microwave detecTors positioned adjacently in the line of motion of the lumber to receive the radiation after transmission through the lumber and consequent attenuation and distortion by the presence of flaws; first and second single-ended operational amplifiers for receiving and amplifying corresponding output signals from said first and second microwave detectors; a differential operational amplifier for detecting and amplifying signal differences between the output signals from said first and second microwave detectors, said signal differences resulting from flaws passing across one of said detectors and said differential operational amplifier being non-responsive to variations in the output signals from said first and second single-ended operational amplifiers due to variations in lumber density affecting said first and second microwave detectors equally; absolute value generation means for inverting negative signal differences but not positive signal differences; signal level sensing means for rejecting those of said signal differences below a selectable threshold level and thereby rejecting variations in the output signals from said first and second single-ended operational amplifiers due to variations in lumber density affecting said first and second microwave detectors substantially equally; and trigger circuit means for generating control pulse signals in response to those of said signal differences above said threshold level.
 16. Apparatus as set forth in claim 15, wherein: said source of microwave radiation generates electromagnetic radiation having a wavelength greater than 1 centimeter; and the flaws to be detected exceed 3 millimeters in diameter.
 17. Apparatus as set forth in claim 15, wherein: said level sensing means includes at least one unijunction transistor; and said trigger circuit means includes at least one silicon controlled rectifier.
 18. A method for detecting flaws in moving lumber of widely varying density, comprising the steps of: transmitting microwave radiation through the lumber; measuring received radiation at at least two adjacent positions substantially equally affected by attenuation variations caused by the widely varying lumber density; sensing significant differences in the radiation measured at said adjacent positions, said differences resulting only from the presence of flaws in the lumber moving through said adjacent positions; and generating in response to said significant differences control signals for use in processing the lumber.
 19. A method as set forth in claim 18, wherein said step of sensing significant differences includes amplifying said differences.
 20. A method as set forth in claim 19, wherein said step of sensing significant differences includes sensing the absolute magnitude of said differences and rejecting those of said differences below a selectable threshold level.
 21. A method as set forth in claim 20, wherein said step of generating control signals includes triggering generation of control pulse signals in response to those of said differences above the threshold level.
 22. A method for detecting flaws in lumber of widely varying density, comprising the steps of: transmitting radiation from a microwave oscillator source; moving the lumber through the microwave radiation; receiving the microwave radiation at two adjacent microwave detectors after transmission through the lumber, the two detectors being substantially equally affected by variations in attenuation of the radiation due to the widely varying lumber density; detecting significant difference signals between outputs from the two detectors, said significant difference signals resulting only from the presence of flaws in the lumber moving across one of the detectors; and generating in response to said significant difference signals control signals for use in processing the lumber.
 23. A method as set forth in claim 22, wherein: said step of Transmitting utilizes a wavelength greater than 1 centimeter; and the flaws to be detected exceed 3 millimeters in diameter.
 24. A method as set forth in claim 22, wherein said step of detecting significant difference signals includes inverting all negative ones of the signal differences, thereby generating absolute values of the signal differences.
 25. A method as set forth in claim 24, wherein said step of detecting significant difference signals includes sensing the absolute values of the signal differences and rejecting those below a selectable threshold level.
 26. A method as set forth in claim 25, wherein said step of generating control signals includes triggering generation of control pulse signals in response to those of said significant difference signals above the threshold level.
 27. A method for detecting flaws in lumber with widely varying density, comprising the steps of: transmitting microwave radiation from a microwave oscillator source; receiving the microwave radiation at two adjacent microwave detectors after transmission through the lumber; moving the lumber between the microwave oscillator source and the two microwave detectors in a direction substantially at right angles to the direction of radiation; amplifying output signals from the two microwave detectors in two corresponding single-ended operational amplifiers; detecting and amplifying in an operational amplifier difference signals representative of the difference between the two output signals, said difference signals resulting from the momentary presence of flaws in the lumber moving across one of the detectors; disregarding gradual fluctuations in lumber density, said fluctuations resulting in practically equal effects on the microwave radiation received at the microwave detectors; inverting all negative ones of said difference signals, thereby generating absolute magnitudes of said difference signals; sensing the absolute magnitudes of said difference signals and rejecting those of said difference signals below a selectable threshold level; and generating control pulse signals for use in processing the lumber, in response to those of said difference signals above the threshold level.
 28. A method as set forth in claim 25, wherein: said step of transmitting utilizes a wavelength greater than 1 centimeter; and the flaws to be detected exceed 3 millimeters in diameter. 